docs(#2171): record the statusline data-source scope boundary (#2178)

Records the statusline scope boundary decided during triage of #2160-2164:
the statusline sources only local, read-only data (refine-existing + new-local),
never credentials or external/network APIs. #2164 (account-usage segment) is
out of scope on this boundary; #2163 (git) is in-scope but on the feature track;
#2160/2161/2162 are approved enhancements.

- docs/adr/2164-statusline-scope-boundary.md (new ADR, Accepted)
- docs/adr/README.md (index row)
- CONTEXT.md (### Statusline glossary/seam entry)
- .out-of-scope/statusline-account-usage.md (#2164 rejection record)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/adr/2164-statusline-scope-boundary.md

@@ -0,0 +1,44 @@
+# Statusline draws its data boundary at local, read-only sources
+
+- **Status:** Accepted
+- **Date:** 2026-07-11
+- **Issue:** #2164
+- **Implementation:** Policy ADR — no code change. Governs triage of statusline enhancement/feature requests.
+
+## Decision
+
+The GSD statusline (`hooks/gsd-statusline.js`) may source data from three tiers, and is bounded to the first two:
+
+| Tier | Data source | In scope? |
+|------|-------------|-----------|
+| 0 — Refine existing | The stdin payload Claude Code already sends (model name, context-window usage, GSD-state read from `.planning/`) | Yes |
+| 1 — New local source | Read-only local reads / bounded subprocesses scoped to the workspace (e.g. `git status`) | Yes, if opt-in and bounded |
+| 2 — External / credentialed | Reading credentials (OAuth tokens, keychains) or calling external/network APIs for data | No |
+
+The statusline refines what it is already handed and may add a new **local, read-only** source, but it does not read credentials or make authenticated/network calls to fetch data. Surfacing account-level or platform-level resource state (usage limits, rate-limit windows) from a GSD hook is a platform concern, not GSD's.
+
+## Rationale
+
+- The statusline is a planning-workflow surface, not a platform dashboard. Its existing segments (model, context meter, directory, GSD-state) are all local, read-only projections of data GSD is already given.
+- Reading credentials from a planning hook is a materially larger trust surface than any rendering concern; even read-only and opt-in, it is not something a planning tool should own.
+- External/undocumented endpoints (e.g. an OAuth usage API) are unstable dependencies that rot silently when they change.
+- Consistent with the existing prior in `.out-of-scope/temporal-context.md`: *"Statusline / TUI re-entry is platform-level, not GSD-level."*
+
+## Consequences
+
+- New statusline requests are triaged against the tier table. The **data-source** axis (this ADR) is orthogonal to the **enhancement-vs-feature** axis (CONTRIBUTING.md): refining an existing segment is an enhancement; adding a new segment or data source is a feature (a new concept/integration), regardless of tier.
+- Applied at decision time:
+  - **#2160 / #2161 / #2162** — refine existing model / context-meter / GSD-state rendering. Tier 0; approved as enhancements.
+  - **#2163** — git segment. Tier 1 (new local source): in scope, but routed to the feature track (`approved-feature` + complete spec) because it adds a new segment.
+  - **#2164** — 5h/7d account-usage segment. Tier 2 (reads OAuth creds + calls `api.anthropic.com/api/oauth/usage`): out of scope; closed `wontfix`, recorded in `.out-of-scope/statusline-account-usage.md`.
+
+## Revisit if
+
+A documented, first-party usage API — or a platform-provided value delivered to the hook without GSD reading credentials — becomes available. That would move usage display out of Tier 2.
+
+## References
+
+- `.out-of-scope/statusline-account-usage.md` — the #2164 rejection record.
+- `.out-of-scope/temporal-context.md` — prior "statusline is platform-level" note.
+- `CONTRIBUTING.md` — enhancement vs feature gates.
+- Issues: #2160, #2161, #2162 (approved enhancements), #2163 (feature-track), #2164 (this ADR's trigger).